1. Field of the Invention
The present invention relates to a liquid crystal display device. In particular, the present invention relates to a liquid crystal display device which has a reduced driving voltage and improved response characteristics and high contrast ratio.
2. Description of the Related Art
A reflection type liquid crystal display device does not require back light since it utilizes ambient light as a light source. Therefore, it is possible to make the most of the advantages of a liquid crystal display device such as being lightweight, having thin body, and low power consumption. However, the brightness of a display screen is limited since the reflection type liquid crystal display device utilizes ambient light as a light source. Moreover, in the case where a polarizing plate, a color filter or the like is used, display brightness is considerably lowered, resulting in a dark display.
In order to solve the above problem, a reflection type STN (Super Twisted-Nematic) mode for optimizing a retardation; as small a number as possible of polarizing plates to be used, for example, one polarizing plate; or a Guest-Host (hereinafter, referred to as GH) mode using a liquid crystal material containing a pleochroic dye (dichroic dye) are conventionally adopted.
In particular, since no polarizing plate is used in a phase-transition type GH (also called White-Taylor type Guest-Host, and hereinafter, referred to as WT) mode utilizing phase transition between a nematicphase and a cholesteric phase, a brighter display is obtained as compared with a liquid crystal display device using other modes.
In a liquid crystal layer of the liquid crystal display device in a WT mode of a certain kind, liquid crystal molecules near the surfaces of substrates are perpendicularly oriented, and a helical pitch of a cholesteric phase is small. Specifically, d/p&gt;2 is satisfied, where a thickness of a cell (thickness of a liquid crystal layer) is d and a helical pitch is p. Hereinafter, the WT mode in which liquid crystal molecules at the interfaces with the substrates are perpendicularly oriented is abbreviated as an HWT (homeotropic WT) mode.
FIG. 3A shows voltage-transmittance characteristics in this HWT mode, and FIGS. 13A through 13C show the orientation states in the HWT mode corresponding to the ranges of applied voltage A, B and C, respectively. FIG. 13A shows the orientation state (initial state) when an applied voltage is within the range A in FIG. 3A, FIG. 13B shows the orientation state when an applied voltage is within the range B in FIG. 3A, and FIG. 13C shows the orientation state when an applied voltage is within the range C in FIG. 3A. In FIGS. 13A to 13C, liquid crystal molecules and pleochroic dyes are not distinguishably shown. FIGS. 13A to 13C show the orientation states of the whole mixture of the liquid crystal molecules and pleochroic dye molecules.
Moveover, as a reflective pixel electrode structure using a two-terminal element which is used in such a reflection type liquid crystal device, there is a reflective pixel electrode structure disclosed in Japanese Laid-Open Patent Publication No. 3-41420. Japanese Laid-Open Patent Publication No. 3-41420 discloses a method for fabricating a reflective pixel electrode by forming a metal film on a flat interlayer insulating film through vapor deposition and then etching the surface of the metal film.
In the case of HWT mode, by mixing an optical active material with liquid crystal molecules, which tend to perpendicularly orientate by themselves, the orientation of the liquid crystal molecules is controlled so that a helical axis of LC molecules in an intermediate region (bulk) of the liquid crystal layer is aligned to be substantially perpendicular to the substrate.
When a voltage is applied to the liquid crystal cell (the liquid crystal layer) of HWT mode, the orientation state changes from that shown in FIGS. 13A to that of 13C passing through an intermediate scattered state shown in FIG. 13B. Due to this change, a threshold voltage is not clearly exhibited. Moreover, a scattered state exists over a wide range B of low voltage as shown in FIG. 3A.
As described above, an initial orientation state is easily disordered and transits to a scattered state at an extremely low voltage. In addition, a decreased helical pitch so as to enhance light-shielding properties results in a high driving signal voltage. Therefore, a dynamic range (a range of voltage levels) of a signal voltage is increased, causing the following problems.
(1) First, in the case of an active matrix type LCD using a thin film transistor (hereinafter, referred to as TFT), a TFT having a high break down voltage (a voltage at which an element is broken) is required to be used. Moreover, the active matrix type LCD is disadvantageous in that the cost of the display device itself is increased due to increase in the signal voltage level.
(2) Second, when a two-terminal element which is advantageous in terms of cost is used as a switching element of an active matrix type LCD, since it is difficult to set a signal voltage applied to a liquid crystal layer at 0 V in an off state, a voltage is undesirably applied to some degree. Thus, in combination of HWT mode in which the initial orientation of liquid crystal molecules is disordered by an extremely low voltage with a two-terminal element, the orientation of the liquid crystal molecules is scattered even in an off state. Therefore, it is difficult to obtain a liquid crystal display device having good display characteristics.
(3) Third, in an HWT mode, the initial orientation state is easily disordered by an extremely small voltage. Thus, when the liquid crystal display device is driven by TFTs, it is impossible to decrease the power consumption by using a driving method, for example, for applying a certain voltage to a liquid crystal layer from an electrode on a side of the counter substrate which does not have an active element so as to decrease the dynamic range of the signal voltage.
Therefore, it is impossible to realize good display quality by applying a switching element such as a TFT and a two-terminal element to a conventional HWT type liquid crystal display mode. Moreover, the conventional liquid crystal display devices are disadvantageous in that various conditions such as a tilt angle of liquid crystal molecules and a ratio of a thickness of the liquid crystal layer to a helical pitch (d/p) should be optimized.
Furthermore, as shown in FIG. 16B, a light reflecting layer 5 is formed on a glass substrate 1b so as to be in contact with a liquid crystal layer 4 as a reflective plate, whereby good display without parallax is obtained. In FIGS. 16A and 16B, a glass substrate 1a on the bottom face of which an ITO film 2 is formed and the glass substrate 1b facing the glass substrate 1a interpose the liquid crystal layer 4. A light beam emitted from a light source 22 is reflected by the light reflecting layer 5 and reaches an observer 26 through an optical path 27.
As the characteristics required for the light reflecting film 5, the following can be listed.
(1) Scattering characteristics to some degree.
(2) Light reflection characteristics having directivity to some degree.
(3) Formation of a height, a size and a pitch of convex portions (not shown) on the surface of light reflecting layer in a random manner so as to avoid coloring.
The reason for the above point (3) is because coloring occurs due to interference between light beams reflected by the adjacent convex portions when a height, a size and a pitch of convex portions are uniformly formed.
In order to satisfy the above conditions, it is effective that the light reflecting layer has a circular surface or an elliptical surface having a longitudinal axis or a diameter of a bottom of a convex portion in the range of 3 to 50 .mu.m. Moreover, it is effective that the distance between the adjacent convex portions is 1 .mu.m or more, and the convex portions having the above shape is placed in a random manner.
Regarding the diameter in a height direction of the convex portion on the surface of the light reflecting layer, since convex and concave portions are formed on the interfaces between the light reflecting layer and the liquid crystal layer, the convex and concave portions on the interfaces induce a nonuniform thickness of the liquid crystal layer. The nonuniformity should be limited within the range where the orientation of the liquid crystal molecules in an AWT (Antiparallel WT) mode are not affected. In this case, the tip of the convex or concave portion of the light reflecting layer should have a circular or an elliptical shape so as to realize good diffuse reflection. The above-mentioned AWT mode denotes a WT mode in which liquid crystal molecules on the interfaces of the substrates are orientated approximately parallel, and is distinguished from the HWT mode.
Furthermore, as the result of converting a margin of a ratio (d/p.sub.0) of a thickness of the cell and a natural pitch of the helical structure into a margin of the thickness of the cell, a margin of the nonuniformity .DELTA.d of the thickness d of the cell is .vertline..DELTA.d .vertline..ltoreq.1.0 .mu.m. Therefore, the central portion in a height direction of the convex portion of the light reflecting layer is set to be an average thickness of the cell. Then, in the range of .+-.0.5 .mu.m from the average thickness of the cell, that is, if a height of the convex portion is 1 .mu.m or less, the light reflecting layer can be formed without adversely affecting the orientation of the liquid crystal molecules.
From the examination based on these experiments, it is desirable that the light reflecting layer has a height of 1 .mu.m or less and a circular or elliptical shape having a diameter or a longitudinal axis of the bottom of the convex portion in the range of 3 to 50 .mu.m and a pitch of the adjacent convex portions of 1 .mu.m or more.
It is difficult to control such strict conditions with high precision by a method for etching the metal film as described in Japanese Laid-Open Patent Publication No. 3-41420.
As methods of an improved WT type liquid crystal display device for driving a liquid crystal display device without using switching elements, there are techniques described in USP. 4596446 and Japanese Laid-Open Patent Publication No. 59-28130. Since the conditions optimal for simple matrix driving are set, only the region where hysterisis does not occur, that is, the region where a twisted angle is in the range of .pi. to 2.pi. rad serves as the liquid crystal display. Moreover, in the case where the liquid crystal display device is used in GH mode containing pleochroic dyes mixed therein, the techniques are disadvantageous in that absorption of light does not sufficiently occur, thereby lowering the contrast.
Furthermore, in the case where the reflective plate is placed, parallax due to the thickness of the glass substrate 1b occurs with a conventional external attachment method, that is, a method for placing the reflective plate 5 on the bottom of the liquid crystal cell, thereby remarkably lowering the viewing characteristics.